Generally, an engine may cause vibrations or noise due to an inertial force generated by a reciprocating motion of a piston. To reduce the vibration or noise, various apparatuses are provided. For example, a cylinder block of the engine may be provided with a balance shaft to reduce the vibrations while the balance shaft rotates in an opposite direction to a crankshaft.
The balance shaft is provided with a driving gear and thus rotates due to a driving force from the crankshaft and may be coupled with a driving/driven transmission gear, an intermediate gear, or the like. Further, the balance shaft is provided with a balance weight for stably rotating the balance shaft and an oil pump for pumping oil.
The oil pump includes an oil pump gear, an oil pump shaft, a housing and a rotor part. The oil pump gear receives a rotating force from a gear of the crankshaft of the engine to rotate while being engaged with the oil pump shaft. The rotor part is press-fitted on an outer circumferential surface of the oil pump shaft and rotates when the pump shaft rotates. Further, the oil pump shaft is press-fitted in an inner side of the rotor part.
The housing includes a support surface extending in the axial direction along the outer circumferential surface of the oil pump shaft and a thrust surface extending radially outwardly from the support surface and contacting both surfaces of the rotor part.
When the engine rotates, an angular acceleration of the engine may be changed. A torque generated therefrom is transferred to the oil pump gear through the crank gear. As a result, the oil pump gear, the oil pump shaft and the rotor part all receive an irregular load. In this case, the oil pump shaft may eccentrically rotate due to torsion or bending. The eccentric rotation of the oil pump shaft is transferred to the rotor part press-fitted on the outer circumferential surface thereof. The rotor part is configured to contact the thrust surface of the housing, and therefore when the oil pump shaft eccentrically rotates, a durability thereof may be lowered or may cause irregular noise.
Meanwhile, according to the related art, the oil pump gear and the rotor are press-fitted in the oil pump shaft and the related art has a structure in which only the rotor is enclosed with the housing, in which the housing is integrally formed with the balance shaft housing. Therefore, a backlash control between the driving gear connecting between the balance shaft fixedly mounted in the balance shaft housing and the oil pump shaft and the oil pump gear may not be made. As such, if the appropriate backlash is not performed between the gears, gear whine noise or a rattle noise may occur.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.